Solution for processing of metal replacement with metal aluminum or aluminum alloy and method for surface processing using such solution

ABSTRACT

There is disclosed a processing solution for metal replacement for metal aluminum or an aluminum alloy. The processing solution is used for surface processing of an underlying aluminum material. The processing solution for metal replacement removes an oxide film on the underlying aluminum material and suppresses corrosive attack to it to allow a plating film having high smoothness and good plating appearance to be deposited on the underlying aluminum material. The processing solution for metal replacement at least includes a metal salt capable of being replaced with aluminum, and an alkaline compound. A quaternary ammonium hydroxide is contained in the processing solution for metal replacement as the alkaline compound.

BACKUNDERLYING MATERIAL OF THE INVENTION

1. Field of the Invention

This invention relates to a solution for metal replacement, for use onmetal aluminum or an aluminum alloy, which solution may be effective inparticular for pre-processing in forming a bump on a wafer, forinstance. This invention also relates to a method for processing thesurface of metal aluminum or the aluminum alloy using the solution formetal replacement.

2. Description of Related Art

Various methods have so far been used for forming an under-bumpmetallization or a bump on a silicon wafer. Among these, there is knowna method consisting in subjecting a aluminum thin film electrode formedby patterning on the silicon wafer, to a processing of zinc replacementfor thereby forming a zinc film, and subsequently forming a bump on theso formed zinc film by electroless nickel plating. There is also known amethod consisting in subjecting the aluminum thin film electrode topalladium processing, in place of the processing of zinc replacement,and subsequently forming a bump by electroless nickel plating. There isfurther known a method consisting in subjecting the surface of thealuminum thin film electrode replace with nickel directly andsubsequently forming a bump by self-catalyzed electroless nickelplating.

No matter which of these methods is used to form an under-bumpmetallization or a bump, the processing of degreasing the aluminum thinfilm electrode or the processing of removing an aluminum oxide film onthe aluminum thin film electrode is generally carried out by way ofpre-processing. In such case, if the aluminum oxide film is an oxidefilm of an extremely thin film thickness, such as is generated by, forexample, immersion in nitric acid, it may directly be subjected toplating by way of post-processing, without raising any particularproblems. However, in case of a rigid aluminum oxide film produced in acertain production process, such as polishing or annealing, is left onthe surface, it may sometimes occur that a plating film, generated in asubsequent process, is insufficient in tight adhesion performance, orthe plating film may be perforated, with the plating film beinginsufficient in adhesion performance. It is therefore strongly desiredthat the rigid aluminum oxide film is completely removed beforehand, andit is also strongly desired that the plating film from the surfaceprocessing process is not liable to be perforated and exhibits highsmoothness.

An example of such processing solution for the above-described surfaceprocessing is a processing solution as disclosed in, for example, theJapanese Laid-Open Patent Publication 2001-316831. This processingsolution contains a zinc compound, an alkali hydroxide, an iron salt,and a chelating agent, such as gluconic acid, for complexing iron ions.In this processing solution, sodium hydroxide, potassium hydroxide orlithium hydroxide is used, either singly or in combination, as theaforementioned alkali hydroxide. Surface processing methods, exemplifiedby a double zincate method, are carried out, using this processingsolution, to remove the oxide film to prevent pitting corrosion toprovide a plating film having high adhesion performance.

-   [Patent Publication 1] Japanese Laid-Open Patent Publication    2001-316831

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The conventional processing solution, described above, contains analkali hydroxide exhibiting strong alkalinity. Hence, the underlyingaluminum material is excessively etched due to strong attack of metalaluminum or the aluminum alloy by this alkaline compound, with theresult that numerous wedge-shaped recesses are formed in the wafersurface. In the subsequent plating film forming process, the nickelplating, for example, is intruded into these recesses to form a platingfilm which is insufficient in surface smoothness, thus detracting fromelectrical conductivity and appearance.

In light of the foregoing, it is an object of the present invention toprovide a solution for metal replacement, for use on metal aluminum oran aluminum alloy, and a method for processing the surface of metalaluminum or the aluminum alloy using the solution for metal replacement.The solution for metal replacement is to be usable for removing an oxidefilm on metal aluminum or an aluminum alloy to improve the adhesionperformance of the plating film. In addition, the solution for metalreplacement is to be low in corrosive attack to the underlying aluminummaterial, and also is to form a plating film exhibiting high smoothnessand good plating appearance.

[Means for Solving the Problem]

The present inventors have conducted eager searches to solve theabove-described problem, and found that, by using a quaternary ammoniumhydroxide as the alkaline compound, contained in the solution for metalreplacement in addition to the salt of metal capable to replacealuminum, it is possible to suppress corrosive attack to metal aluminumor to an aluminum alloy.

The present inventors have also found that, by contacting a material forprocessing, containing metal aluminum or an aluminum alloy on itssurface, with the processing solution for metal replacement containing aquaternary ammonium hydroxide as an alkaline compound, removing an oxidefilm on metal aluminum or the aluminum alloy and performing theprocessing of metal replacement of replacing the aluminum with themetal, contained in the processing solution, for aluminum, therebydepositing a film of the metal, it is possible to remove an oxide filmadhered to the underlying aluminum material as well as to process thesurface of the underlying aluminum material in readiness for depositinga plating film having good plating appearance.

The present invention provides a processing solution for metalreplacement, for use on metal aluminum or an aluminum alloy, containingat least a salt of metal capable to replace aluminum, and an alkalinecompound, in which, according to the present invention, the alkalinecompound is a quaternary ammonium hydroxide.

The present invention also provides a method for surface processing ofmetal aluminum or an aluminum alloy. The method comprises the step ofcontacting a material for processing, containing metal aluminum or analuminum alloy on its surface, with a processing solution for metalreplacement containing a quaternary ammonium hydroxide as an alkalinecompound, removing an oxide film on metal aluminum or the aluminumalloy, and performing the processing of metal replacement of replacing aaluminum with a metal contained in the processing solution for metalreplacement to form a film of the metal.

With the processing solution for metal replacement for metal aluminum orthe aluminum alloy, according to the present invention, the alkalinecompound contained in the solution is the quaternary ammonium hydroxide.It is thereby possible to suppress corrosive attack to the underlyingaluminum material to suppress cracking.

Furthermore, with the method for surface processing of metal aluminum oran aluminum alloy with the use of the processing solution for metalreplacement according to the present invention, it is possible toperform pre-plating surface processing of removing an oxide film adheredto the underlying aluminum material and for depositing a plating filmhaving good appearance and high smoothness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solution for metal replacement for metal aluminum or an aluminumalloy, and the method for processing the surface of metal aluminum orthe aluminum alloy using the solution for metal replacement, accordingto the present invention, will now be described in detail.

The processing solution for metal replacement according to a preferredembodiment of the present invention at least contains a metal salt thatmay be replaced with aluminum, and an alkaline compound. The processingsolution contains a quaternary ammonium hydroxide as an alkalinecompound

The metal that forms the metal salt contained in the processing solutionfor metal replacement according to the present embodiment is a metalthat may be replaced for aluminum. Such metal exhibits the ionizationtendency lower than aluminum, and may be enumerated by zinc, palladium,nickel, iron, cobalt, tin, zinc, lead, copper, silver, gold andplatinum. The metal salts used may be oxides, sulfates, chlorides orgluconates of these metals. Specifically, zinc oxide, zinc sulfate, zincchloride or zinc gluconate may be used. One or more of these metal saltsmay be used in desired proportions for the processing solution for metalreplacement of the present embodiment.

Although there is no particular limitation to the concentration of themetal salt, the metal amount is normally not less than 1 ppm andpreferably not less than 10 ppm. An upper limit of the concentration ofthe metal salt is normally not higher than 100,000 ppm and preferablynot higher than 20,000 ppm. If the metal salt concentration is too low,the metal may not be sufficiently replaced with the underlying aluminummaterial, or it may become necessary to supply additional amounts ofmetal salts. On the other hand, if the metal salt concentration is toohigh, with metal aluminum or the aluminum alloy being an electrodepatterned on a wafer, there are cases where a part other than underlyingmetal aluminum or aluminum alloy material is eroded, or where the metalsalt seeps to a site other than underlying aluminum or aluminum alloymaterial so as to be precipitated thereon.

The processing solution for metal replacement of the present embodimentcontains quaternary ammonium hydroxides as an alkaline compound.Although there is no limitation to the quaternary ammonium hydroxidesused, these may, for example, be quaternary ammonium hydroxidescontaining alkyl and/or hydroxyalkyl groups with 1 to 4 carbon atoms,such as tetramethylammonium hydroxide (TMAH), tetraethylammoniumhydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide,trimethyl (2-hydroxyethyl) ammonium hydroxide (choline) or triethyl(2-hydroxyethyl) ammonium hydroxide. From the perspective of the oxidefilm removing effect, stability and cost, tetramethylammonium hydroxide(TMAH) and trimethyl (2-hydroxyethyl) ammonium hydroxide (choline) aremost preferred. These alkaline compounds may be used either alone or incombination. If the alkaline compounds are used in combination, thesemay be used in desired optional proportions.

Although there is no particular limitation to the concentration of thequaternary ammonium hydroxides, as the alkaline compounds, it is on theorder of 100 g/L to 1,000 g/L. It may optionally be desirable to changethe concentration depending on whether only one type of the quaternaryammonium hydroxides is used or at least two types of the quaternaryammonium hydroxides are used in combination. With the processingsolution for metal replacement for use on metal aluminum or an aluminumalloy of the present embodiment, containing the quaternary ammoniumhydroxide instead of an alkali hydroxide, as alkaline compound, it ispossible to prevent corrosive attack to the underlying aluminum materialto render it possible to carry out optimum pre-processing in a mannerfree from cracking.

The processing solution for metal replacement may further contain aniron salt or salts. By the processing solution for metal replacementcontaining an iron salt or salts, it is possible to form a dense film bymetal replacement, such as a dense zinc film, on metal aluminum or on analuminum alloy. Examples of the iron salts include iron chloride, ironsulfate, iron nitrate and iron gluconate, only by way of illustration.These iron salts may be used either alone or in combination. If theseiron salts are used in combination, these may be used in desiredoptional proportions. The concentration of the iron salts may be in arange from 0.1 to 100 mmol/L and preferably in a range from 0.5 to 50mmol/L.

The processing solution for metal replacement may further contain acomplexing agent. The complexing agent, contained in the processingsolution for metal replacement, may form a complex with iron ions incase the aforementioned iron salt is contained in the solution. With thecomplexing agent thus forming a complex with iron ions, it is possibleto suppress pitting corrosion of the underlying aluminum material by theiron ions. The complexing agent may be general complexing agents orchelating agents, and may, for example, be hydroxycarboxylic acid, suchas glycolic acid, lactic acid, malic acid, tartaric acid, citric acid,gluconic acid or glucoheptonic acid, and salts thereof, aminocarboxylicacids, such as glycine, aminodicarboxylic acid, nitrilotriacetic acid,EDTA, hydroxyethyl ethyleneamine triacetic acid,diethylenetriaminepentaacetic acid or polyaminopolycarboxylic acid, andsalts thereof, phosphorous acid-based chelating agents, such as HEDP,aminotrimethylphosphonic acid or ethylenediamine tetramethyl phosphonicacid, and salts thereof, amine-based chelating agents, such asethylenediamine, diethylenetriamine or triethylenetetramine, and saltsthereof The concentration of the complexing agent may, for example, bein a range from 0.5 to 100 g/L and preferably in a range from 1 to 50 g/L in case of using tartaric acid as the complexing agent.

The processing solution for metal replacement may further contain sodiumnitrate, as necessary. Sodium nitrate contained in the processingsolution for metal replacement, acts in conjunction with the iron ionsto improve characteristics of the film of the metal. Specifically, theconcentration of the complexing agent may be in a range fromapproximately 0.01 to 10 g/L and preferably in a range fromapproximately 1 to 5 g/L.

In addition, surface active agents may be contained in the processingsolution for metal replacement of the present embodiment with a view toimproving the capacity of removing oxide films and providing for waterwettability. The surface active agents may be enumerated by, forexample, nonionic surfactants, such as polyethylene glycol orpolyoxyethylene oxypropylene block copolymers, anionic surfactants, suchas dodecylbenzene sodium sulfonate, polyoxyethylene laurylether sodiumsulfate or polyoxyethylene nonyl phenylether sodium sulfonate, andcationic surfactants. These surfactants may be used either alone or incombination. If these surfactants are used in combination, these may beused in desired optional proportions. The concentration of thesurfactants may be in a range from 1 to 10,000 ppm, preferably in arange from 5 to 5,000 ppm and more preferably in a range from 10 to2,000 ppm.

Preferably, the processing solution for metal replacement of theembodiment described above is prepared as an aqueous solution from theperspective of operational safety. However, it is also possible to useother solvents, such as methanol, ethanol, ethylene glycol, diethyleneglycol, triethylene glycol, glycerin, IPA or mixed solvent with thewater. These solvents may be used either alone or in combination. If thesolutions are used in combination, these may be used in desired optionalproportions.

The method for processing the surface of metal aluminum or the aluminumalloy using the processing solution for metal replacement of theembodiment of the present invention will now be described in detail.

With the present surface processing method, a material for processing,at least including metal aluminum or an aluminum alloy on its surface,is brought into contact with the aforementioned processing solution formetal replacement. This removes an oxide film adhered to metal aluminumor to the aluminum alloy. Aluminum is replaced with a metal contained inthe processing solution for metal replacement is replaced with aluminumto form a film of the metal on the surface of the material forprocessing. This surface processing method is a pre-processing method tobe carried out in advance of processing of forming a plating film, suchas a nickel plating film or a palladium plating film, on the materialfor processing. More specifically, the processing solution for metalreplacement is brought into contact with the material for processing,having at least metal aluminum or the aluminum alloy on its surface, toremove the oxide film adhered to the surface of the material to increasethe adhering performance of, for example, a nickel plating film in thecourse of the subsequent process.

The alkaline compound, contained in the processing solution for metalreplacement of the present embodiment, is the quaternary ammoniumhydroxide, as described above. It is thus possible with the presentprocessing solution for metal replacement to prevent excessive attack tothe underlying aluminum material by an alkali hydroxide, as compared tothe case of using the conventional processing solution for metalreplacement containing the alkali hydroxide, such as sodium hydroxide orpotassium hydroxide. In this manner, it is possible to generate aplating film of high smoothness and good surface appearance. Inaddition, from the perspective of safety, the present processingsolution for metal replacement may be handled more easily than theconventional processing solution for metal replacement which neededmeticulous attention in handling. Hence, disposal of drainage water maybe done in a manner that makes much of environment protection.

A method for surface processing of metal aluminum or an aluminum alloy,more specifically, a method for surface processing of metal aluminum oran aluminum alloy by a processing solution for metal replacement thatcontains zinc as a metal species capable of being replaced withaluminum, that is, a zincate processing solution, is now described indetail. The zincate processing solution, used here, is an alkalinesolution containing zinc ions and also containing a quaternary ammoniumhydroxide as an alkaline agent. With the zincate processing solution, anoxide film adhered to the material for processing, including metalaluminum or an aluminum alloy at least on its surface, is removed, andparticles of zinc are precipitated on the surface of the material forprocessing as a result of the replacement reaction brought forth due tothe difference between the electrode potential of zinc and that ofaluminum. The material for processing is sometimes referred to below asan aluminum substrate. This method for surface processing, a processingcarried out prior to plating on the aluminum substrate with the use ofthe zincate processing solution, is carried out in general as a doublezincate process. This double zincate process includes (1) first zincreplacement processing of the aluminum substrate, (2) acid cleaning and(3) second zinc replacement processing. The double zincate process isfollowed by (4) plating processing, such as electroless nickel plating.This double zincate processing is also explained in the followingdetailed explanation of the surface processing method. It should benoted that the surface processing method that uses the above-describedprocessing solution for metal replacement is not limited to this doublezincate processing, and single or triple zincate processing may also beused within the scope of the present invention.

(1) First zinc Replacement Processing

The aluminum substrate, as the material of plating processing, includesmetal aluminum or an aluminum alloy at least on its surface, and may beprepared by coating an aluminum layer on a non-aluminum material, suchas a silicon plate, by any suitable method, such as sputtering. Thealuminum layer may be coated on all or part of the non-aluminummaterial, and may be of a thickness which is ordinarily not less than0.5 μm and preferably not less than 1 μm. In preparing the aluminumsubstrate, a method of vacuum evaporation or an ion plating method mayalso be used in place of sputtering. It should be noted that metalaluminum or an aluminum alloy, existing at least on the surface of thealuminum substrate, used herein, may not only be pure industrialaluminum A1100 according to JIS standard, or a highly corrosionresistant alloy, but may also be a highly corrosive alloy. For example,blank sheets, a rolled material or a casting may be used satisfactorily.There is no limitation to the shape of metal aluminum or an aluminumalloy, such that materials of variable shapes, such as those of aplate-like or rectangular shape, may be used. There is also nolimitation to the composition of metal aluminum or an aluminum alloy.That is, the method for surface processing, employing the processingsolution for metal replacement of the present embodiment, may be usedfor an underlying aluminum material of, for example, the Al—Si or Al—Cucomposition.

Initially, this aluminum substrate is subjected to cleaning, such asdegreasing, by any suitable conventional method. It is then washed withwater and subjected to etching, as conventionally, using an alkali or anacid. Specifically, the degreasing processing is carried out byimmersion in a degreasing solution for aluminum, or by electrolyticdegreasing. On the other hand, the etching processing is carried out byimmersing the aluminum substrate in an alkaline solution ofapproximately 1 to 10% or an acidic solution of approximately 1 to 20%,at a liquid temperature of approximately 40 to 70° C. for about 1 to 15minutes.

The so processing aluminum substrate is immersed in an acidic solutionfor a preset time for removing etching residues (smuts) by an alkali oran acid. More specifically, the aluminum substrate, etched as describedabove, is immersed in an aqueous solution of nitric acid, with aconcentration of nitric acid of approximately 200 to 700 ml/L andpreferably approximately 450 to 550 ml/L, for approximately 30 secondsto 2 minutes, thereby removing the smuts.

The aluminum substrate, deputed as described above, is then washed withwater and immersed in a zincate processing solution (processing solutionfor metal replacement) by way of performing the first processing of zincreplacement. This zincate processing solution is an alkaline zincic acidsolution containing a quaternary ammonium hydroxide. Specifically, thealuminum substrate is immersed in the zincate processing solutionincluding the above composition with a liquid temperature of 10 to 50°C. and preferably 15 to 30° C. If the temperature of the zincateprocessing solution is not less than 10° C., the replacement reaction isnot excessively retarded such that the zinc film free from surfaceirregularities may be formed. If the temperature of the zincateprocessing solution is not higher than 50° C., the replacement reactionis not excessively promoted such that it is possible to prevent the zincfilm from becoming roughed. The aforementioned temperature range istherefore desirable.

There is further no limitation to the time of immersion such that it maybe optionally set by taking into account the thickness of the film ofaluminum oxide to be removed. For example, the time of immersion mayordinarily be not less than five seconds and preferably not less thanten seconds, with the upper limit being five minutes or less. If thetime of immersion is too short, the process of replacement is retardedand the oxide film may be removed only insufficiently, whereas, if it istoo long, there is the risk that the processing solution is intruded viasmall holes in the replaced metal layer to permit metal aluminum or analuminum alloy to become dissolved and discharged. These points need tobe taken into account in setting the time of immersion.

By immersing the aluminum substrate in the zincate processing solution,in this manner, it is possible to remove the oxide film adhered to thesubstrate.

(2) Processing of Acid Cleaning

The aluminum substrate, thus immersed in the zincate processingsolution, is rinsed with cold water, and is then immersed in an acidicsolution having an oxidative effect, such as an aqueous solution ofnitric acid, an acidic solution not having an oxidative effect, such ashydrochloric acid or sulfuric acid, or in an aqueous solution preparedby adding hydrogen peroxide or sodium persulfate, having an oxidativeeffect, to the acidic solution, such as hydrochloric acid or sulfuricacid. This peels off and removes the film of the zinc. If an aqueoussolution of nitric acid is used as the acidic solution, such solutionhaving the concentration of nitric acid of approximately 350 to 600 ml/Land preferably approximately 450 to 550 ml/L may be used. The aqueoussolution of nitric acid may contain iron ions, as also disclosed in theU.S. Pat. No. 5,141,778. The aqueous solution of nitric acid, having aliquid temperature of approximately 15 to 30° C., for example, may beused, and the aluminum substrate may be immersed therein forapproximately 30 to 60 seconds to remove the film of zinc. During thisprocessing, the aluminum substrate may be standstill or the solution maybe stirred.

(3) Second Zinc Replacement Processing

After the film of the zinc by the first zinc replacement is removed byimmersion in the acidic solution, this aluminum substrate is washed withwater, and the second zinc replacement processing is carried out. Forthe second processing of zinc replacement, the zincate processingsolution of the same composition as that of the first processing of zincreplacement may be used. Although the conditions for processing, such asprocessing time or processing temperature, may be the same as those ofthe first processing of zinc replacement, the processing time for thesecond processing of zinc replacement may be longer by about 1 to 60seconds than that for the first processing of zinc replacement. Ofcourse, the composition of the processing solution for the secondprocessing of zinc replacement may be different from that of theprocessing solution for the first processing of zinc replacement. If itis desired to reduce the film thickness of the film of the zinc by thesecond processing of zinc replacement, the composition of the processingsolution may desirably be changed such as by decreasing the zinc ionconcentration.

Thus, the aluminum substrate is subjected to the first processing ofzinc replacement, using the zincate processing solution, and thenimmersed in the acidic solution, such as a nitric acid solution toremove the film of the zinc. The second processing of zinc replacementis then carried out to remove the oxide film adhered to the surface ofthe substrate. A film of the zinc is further deposited to activate thesurface of the substrate to enable an optimum plating film to bedeposited on the material for processing.

Although the processing by the double zincate method has been describedabove, it is also possible to form plating film such as electrolessnickel plating after having performed the first processing of zincreplacement, or to form plating film after having removed the film ofthe zinc. The latter alternative is preferred from the perspective ofpositively removing the oxide film and from the perspective of improvingthe density of the plating film.

(4) Processing of Plating

This processing of plating is carried out by subjecting thezincate-processed aluminum substrate to electroless plating orelectrolytic plating. For example, plating is performed to an ultimatefilm thickness, using a proper metal plating solution, such as a platingbath of electroless nickel plating, electroless palladium plating or acopper plating.

Specifically, electroless nickel plating is now described as an example.With the electroless nickel plating solution, nickel ions are affordedby the use of water-soluble nickel salts, such as nickel sulfate, nickelchloride or nickel acetate, with the concentration of nickel ions beingapproximately 1 to 10 g/L. The electroless nickel plating solutioncontains a complexing agent for nickel, such as ammonium salts or aminesalts, or organic acid salts, such as acetates, succinates or citrates,in a concentration range of approximately 20 to 80 g/L. The electrolessnickel plating solution also contains hypophosphous acid orhypophosphites, such as sodium hypophosphite, with the concentrationrange from approximately 20 to 40 g/L, as a reducing agent. With theplating solution containing e.g. hypophosphites, for example, as areducing agent, it is possible to elevate the stability of the platingsolution to render it possible to deposit an inexpensivenickel-phosphorus alloy film. The plating solution composed of thesecompounds is prepared so that the pH value will be approximately 4 to 7.Plating with the plating solution is carried out by immersing thealuminum substrate in the plating solution for approximately 15 secondsto 120 minutes, as the plating solution is adjusted to a temperature of80 to 95° C. The thickness of the plating film may be changed byproperly changing the plating time duration.

The plating is not limited to the electroless plating and may also beelectrolytic plating. Plating metals may also be Cu or Au, in place ofthose shown above. Plating may also be carried out in such a way thattwo or more layers will be formed by, for example, immersion platingmethod.

It should be noted that the processing conditions or concentrationsetting in the zincate processing or plating processing are not limitedto those shown above, and may properly be changed depending on e.g. thethickness of the film being formed.

If, in carrying out the surface processing, described above, theprocessing solution for metal replacement of the present embodiment, inwhich the quaternary ammonium hydroxide is contained as an alkalinecompound in place of an alkali hydroxide, is used, it is possible toremove the aluminum oxide film, adhered to the underlying aluminummaterial, as well as to reduce corrosive attack to the surface of thealuminum substrate as the material of processing. Hence, a plating filmwith high smoothness and good appearance may be generated by subsequentplating processing without excessive etching.

It should be noted that the conventional processing solution for metalreplacement, containing an alkali hydroxide, such as sodium hydroxide,needs to be handled only with meticulous attention. Conversely, theprocessing solution for metal replacement of the present embodiment, inwhich the alkali hydroxide is not contained and the quaternary ammoniumhydroxide is contained as the alkaline compound, can be handled withease. Furthermore, the processing solution for metal replacement of thepresent embodiment can be discharged with ease as compared to theconventional processing solution for metal replacement which can bedischarged extremely onerously. Thus, with the processing solution formetal replacement of the present embodiment, it may be said that theaspect of environmental protection has duly been taken into account.

It should also be noted that, in surface processing with theconventional processing solution for metal replacement, containing analkali hydroxide, changes in temperature severely affect the process ofremoval of the oxide film or deposition of the film. With the use of thepresent processing solution for metal replacement, containing thequaternary ammonium hydroxide as the alkaline agent, the processing ofmetal replacement may be carried out at an ambient temperature, that is,without the necessity of using a cooling device or the like, therebyreducing equipment cost and shortening the processing time.

The present invention in not limited to the above-described embodimentsuch that design changes which do not depart from the purport of thepresent invention may be made within the scope of the invention.

Certain specified Examples of the present invention and a ComparativeExample are now described hereinbelow.

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLE

A silicon plate, coated with an aluminum layer to a thickness of 5 μm bya sputtering method, was immersed with cleaning and etched, asconventionally, for use as a material for plating processing.

This material for plating processing was immersed for one minute in anaqueous solution of nitric acid, with a concentration of 500 ml/L, forsmut removal. The so processed material for plating was then immersed inan alkaline zincic acid solution, having the composition shown in Table1, shown below, in order to carry out alkaline zinc replacementprocessing. The so processed material was then immersed in an aqueoussolution of nitric acid, with the concentration of 500 ml/L, at 25° C.for one minute, in order to peel off and remove the film of the zinc.The so processed material was then again immersed in an alkaline zincicacid solution, prepared in accordance with the composition, shown inTable 1 below, in order to carry out the processing of alkalireplacement in similar manner (double zincate method).

The resulting material was then subjected to nickel plating by theelectroless plating method to a film thickness of 0.5 μm, and then togold plating on the so formed nickel plating by the immersion platingmethod to a thickness of 0.05 μm.

The resulting plated material was visually checked as to appearance, andevaluation was made of the state of the plating film. Since theelectroless nickel plating film was formed to a thin thickness and thegold plating film was formed thereon, neither nickel nor gold was notprecipitated and a hole (colored in white) was left if the oxide filmwas not removed but left. Thus, the state of the plating film could beevaluated by evaluating the state of non-adhesion (the left-over stateof the oxide film) as contrasted to the gold color. Further, across-section of the plated material was formed by a focused ion beam(FIB) method to visually check the etched state of the underlyingaluminum material. The etched state was visually checked by takingadvantage of the fact that, if the underlying aluminum material isetched to a recessed shape, nickel is intruded into the recess and maythus be observed as a shaped of spike. The results are shown in thefollowing Table 1.

TABLE 1 Compar- concentration of the ative ingredients in 1 L ofExamples Example water 1 2 3 1 zincate alkali TMAH 900 450 process-(base) (25%) ing (g/L) solution Choline 600 300 (50%) (g/L) NaOH 100(g/L) zinc oxide ZnO (g/L) 12 12 12 12 gluconic (g/L) 20 20 20 20 acid(50%) iron (g/L) 5 5 5 5 chloride solution ° C. 25 25 25 20 tempera-ture Time sec. 60 60 60 60 presence or absence of absent absent absentpresent alkali metal appearance as visually checked good good good goodFIB check; Ni spikes small small small many

It is seen from the results of the above Table 1, in case surfaceprocessing is carried out using the zincate processing solution of thepresent embodiment, the oxide film, adhered to the underlying aluminummaterial, has been removed satisfactorily, while Ni spikes are scarcelynoticed, and the state of etching of the underlying aluminum material isgood, thus indicating that the corrosive attack has been suppressedsufficiently.

Conversely, with surface processing with the use of a zincate processingsolution, containing sodium hydroxide as an alkaline agent, there werenoticed numerous nickel spikes on the underlying aluminum material, eventhough the oxide film adhered to the underlying aluminum material, couldbe removed. It was thus seen that the underlying aluminum material wasexcessively etched due to the strong corrosive attack from the alkalineagent.

It is seen from these results that the zincate processing solution ofthe present embodiment has a high oxide film removing performance andexhibits only low corrosive attack to the underlying aluminum material.

1. A processing solution for metal replacement, for use on metalaluminum or an aluminum alloy, the processing solution at leastcontaining a metal salt capable of being replaced for aluminum, and analkaline compound, wherein the alkaline compound is a quaternaryammonium hydroxide.
 2. The processing solution for metal replacementaccording to claim 1 wherein the quaternary ammonium hydroxide containsan alkyl group and/or a hydroxyalkyl group.
 3. The processing solutionfor metal replacement according to claim 1 wherein the quaternaryammonium hydroxide is tetramethylammonium hydroxide and/or trimethyl(2-hydroxyethyl) ammonium hydroxide.
 4. The processing solution formetal replacement according to claim 1 wherein the metal is zinc.
 5. Amethod for surface processing of metal aluminum or an aluminum alloycomprising the steps of: contacting a material for processing,containing metal aluminum or an aluminum alloy on a surface thereof,with the processing solution for metal replacement according to claim 1;removing an oxide film on the metal aluminum or aluminum alloy; andperforming the processing of metal replacement of replacing the aluminumwith the metal contained in the processing solution for metalreplacement to form a film of the metal.
 6. The method for surfaceprocessing of metal aluminum or an aluminum alloy according to claim 5in which, after forming the film of the metal, a plating film isdeposited on the surface of the film of the metal.
 7. The method forsurface processing of metal aluminum or an aluminum alloy according toclaim 5 in which, after forming the film of the metal, the material forprocessing is immersed in an acidic solution having an oxidative effectto remove the film of the metal.
 8. The method for surface processing ofmetal aluminum or an aluminum alloy according to claim 7 wherein, afterremoving the film of the metal by the acidic solution having theoxidative effect, a further processing of metal replacement is carriedout to deposit another film of the metal on the same surface of thematerial for processing.
 9. The method for surface processing of metalaluminum or an aluminum alloy according to claim 5 wherein the metal iszinc.
 10. A method for surface processing of metal aluminum or analuminum alloy comprising the steps of: contacting a material forprocessing, containing metal aluminum or an aluminum alloy on a surfacethereof, with the processing solution for metal replacement according toclaim 2; removing an oxide film on the metal aluminum or aluminum alloy;and performing the processing fo metal replacement of replacing thealuminum with the metal contained in the processing solution for metalreplacement to form a film of the metal.
 11. A method for surfaceprocessing of metal aluminum or an aluminum alloy comprising the stepsof: contacting a material for processing, containing metal aluminum oran aluminum alloy on a surface thereof, with the processing solution formetal replacement according to claim 3; removing an oxide film on themetal aluminum or aluminum alloy; and performing the processing fo metalreplacement of replacing the aluminum with the metal contained in theprocessing solution for metal replacement to form a film of the metal.12. A method for surface processing of metal aluminum or an aluminumalloy comprising the steps of: contacting a material for processing,containing metal aluminum or an aluminum alloy on a surface thereof,with the processing solution for metal replacement according to claim 4;removing an oxide film on the metal aluminum or aluminum alloy; andperforming the processing fo metal replacement of replacing the aluminumwith the metal contained in the processing solution for metalreplacement to form a film of the metal.